Stargazing: Alpha, Beta, Gamma, Delta, Who?

Stargazers identify stars and other objects in relation to which constellation they’re in. No matter what you’re looking for in the night sky, you can find it in one of the 88 official constellations. Star maps or astronomy guidebooks often list the names of stars in a seemingly baffling array of numbers, letters and abbreviations. If you’re going to learn your way around the sky, it helps to know how these names are given.

The 88 constellations are all given three-letter abbreviations that help identify them. Stars that lie within a constellation boundary are referred to with these three-letter codes. In addition, stargazers use what’s called the genitive form of the constellation name. For example, the genitive form of Orion is Orionis, so if you’re referring to a star in Orion, you would call it ‘something Orionis’.

The brightest stars in any constellation are usually the ones that are used to make up the pattern, and many of the bright stars stand out so much that they’ve been given proper names. Regardless of whether a star has a proper name or not, though, the bright ones all have Greek letters associated with them.

Bayer letters

The brightest star in any constellation is usually given the designation alpha, represented by the Greek letter α, while the second brightest is beta, β, and so on. These Greek letters are known as the Bayer letters.

You can usually tell how bright a star is relative to the others in the same constellation by comparing Bayer letters: those stars with a Bayer designation near the beginning of the Greek alphabet are brighter than those who languish at the tail end of the alphabet.

When a star is bright enough to be given its own Bayer letter, that Greek letter is added as a prefix to the three-letter constellation abbreviations. So if you see a star named as α, many times it’s the brightest one in that constellation.

(Actually, this generalisation isn’t always the case. Orion makes a good illustration of an exception to the rule: in Orion, the brightest star, Rigel, or β Orionis, is a little brighter than Betelgeuse, α Orionis!)

The Greek alphabet

Alpha, á

Iota, é

Rho, ñ

Beta, â

Kappa, ê

Sigma, ó

Gamma, ã

Lambda, ë

Tau, ô

Delta, ä

Mu, ì

Upsilon, õ

Epsilon, å

Nu, í

Phi, ϕ

Zeta, æ

Xi, î

Chi, ÷

Eta, ç

Omicron, ï

Psi, ø

Theta, è

Pi, ð

Omega, ù

It’s not really possible to compare the brightness of stars between constellations using their Bayer letters, because the system relates to the relative brightness of stars within a constellation. For example, the brightest stars in Orion are α Orionis and β Orionis, and the dimmer ones include stars with middling Bayer letters, such as μ Orionis. But μ Orionis is brighter than the nearby α Monocerotis, the brightest star in the constellation Monoceros!

Proper names

Some of the brightest stars have a proper name, which you can use interchangeably with the star’s Bayer designation. For example, the brightest star in the constellation Carina is called Canopus, and it also has the Bayer designation α Carinae. The names given to stars come from a variety of different cultures and languages, but most stars have proper names from Latin, Greek or Arabic.

Many of the stars in the southern hemisphere lack proper names because they weren’t visible to the Greek and Arabic stargazers who catalogued the stars centuries ago. That’s not to say that people in the southern hemisphere didn’t name their own stars, but rather that astronomers use a rather restricted list of named stars, those formally recognised by the IAU.

The ten brightest stars in the sky all have proper names, and they are, in order of brightness: